1. Technical Field
The present invention relates generally to hot plugging of peripheral devices and in particular to hot plugging peripheral devices to a bus operating selectively in low voltage differential mode, high voltage differential mode, or single-ended mode. Still more particularly, the present invention relates to preserving the integrity of data signals through adaptable precharging when hot plugging a peripheral device to a bus operating in a selectable transmission mode.
2. Description of the Related Art
Many modern data processing employ buses conforming to the small computer system interface (SCSI) standard to connect disk drives or redundant array of inexpensive disk (RAID) devices to a system bus. The replacement of peripheral devices, such as disk drives in personal computers or RAID boxes, connected to a SCSI bus during a data transfer on that bus is called "hot plugging." Since the device being connected to the bus is normally without power, external pins in the device represent discharged capacitors. At the instant when the pin comes in contact with a transmission line within the bus (a bus conductor), the pin capacitor will act as an ideal short to ground. This will disrupt the signal level at the instant of connection, which may cause an interruption of the data transfer.
An example of the problem caused by hot plugging peripheral devices to a SCSI bus is depicted in FIG. 6. The example depicted relates to low voltage differential (LVD) transmission in a SCSI environment, in which a differential signal between a pair of cable wires carrying two signals V.sub.N and V.sub.A, centered around a reference voltage V.sub.REF of approximately 1.25 V, has a magnitude which is typically about 400 mVpp (millivolts, peak-to-peak). At a time t.sub.1, a peripheral device pin makes contact with the cable connector and pulls the signal level to ground. The differential signal V.sub.N -V.sub.A changes polarity, which may be interpreted by the receiver as an ordinary data transition, resulting in a data transfer error.
The prior art typically addresses the problems associated with hot plugging after the fact, by re-transmission of data after the interruption. However, the standard SCSI connector SCA-2 provides a mechanical means, through pins of different lengths, for connecting the ground and power supply bus conductors to the corresponding signal pins on the SCSI board or other peripheral device before the data signal conductors are connected to the corresponding data signal pins. This mechanism may be employed, as suggested in the prior art, to precharge the signal pins on a SCSI card during hot plugging. Typically, however, such precharging is concerned with power surges during hot plugging and not with preserving data integrity. Therefore, most conventional precharging schemes propose precharging the pins to a fixed, predefined voltage to limit current and power consumption during hot plugging.
Precharging pins to a fixed, predefined voltage may limit current consumption during hot-plugging but does not necessarily preserve data integrity since actual signal voltages during operation are not fixed and may vary across a wide range defined tolerances. In the case of LVD SCSI, for instance, the actual reference voltage V.sub.REF around which the data signals V.sub.N and V.sub.A are centered may vary from 0.7 V to 1.8 V, with the typical value being 1.25 V. The tolerance for the reference voltage V.sub.REF is thus more than twice the magnitude of the differential signal .vertline.V.sub.N -V.sub.A .vertline..ltoreq.400 mV. Any predefined precharge voltage having a constant or fixed value will not necessarily prevent changes in the differential signal which may being interpreted by a receiver as a data transition.
Additionally, some devices are designed to selectively operate in more than one transmission mode. A single device, such as the Symbios Model 53C895 Universal Transceiver available from Symbios, Inc. of Fort Collins, Colo., may be configured to operate in either the LVD SCSI mode, the high voltage differential SCSI mode, or the single ended (SE) SCSI mode, depending on the transmission mode of the bus to which the device is connected. The different transmission modes employ different voltages and voltage ranges. For example, LVD SCSI employs a differential signal centered on a reference signal somewhere in the range of 0.7 V to 1.8 V, as noted above, which SE SCSI employs a voltage swing of 0 V to 3.3 V.
Even the single ended SCSI transmission mode is sensitive to data signal fluctuations during hot-plugging. SE SCSI signals swing from V.sub.SS to V.sub.TERM, as defined by the external terminator and loaded bus. To improve noise immunity, the front-end of an SE SCSI receiver may be implemented in the form of a Schmitt Trigger with hysteresis centered around the TTL trip point of 1.4 V. Thus, for a typical hysteresis window of about .+-.150 mV, the trip point is 1.55 V for the positive signal transition and 1.25 V for the negative signal transition. Fluctuations of the data signal conductor voltages during hot-plugging may cross these trip points. The hysteresis may also be variable. Therefore, a fixed precharge voltage will not suffice to preserve data integrity.
It would be desirable, therefore, to provide adaptable precharging of data signal pins based on actual operating voltages for preservation of data integrity. It would further be advantageous for the adaptable precharging to automatically accommodate any of several selectable transmission modes.